As a class, the recently discovered third allotropic form of carbon is termed fullerenes, and the prototypical form of this class is a spherical hollow molecule containing 60 carbon atoms, i.e., C.sub.60, that is termed buckminsterfullerene (Kratschmer, W., et al., Nature 347:354-357 (1990); Ajie, H. et al., J. Phys. Chem., 94:8630-8633 (1990); and Diederich, F., et al., Science. 252:548-551 (1991)). The fullerene family includes asymmetrical forms, such as C.sub.70, as well as cylindrical fibers nicknamed buckytubes. In C.sub.60, hexagons and pentagons of carbon link together in a coordinated fashion to form a hollow, geodesic dome with bonding strains equally distributed among 60 carbon atoms. Some of the electrons are delocalized over the entire molecule, similar to benzene. However, benzene is flat and many of its derivatives tend to stack in flat sheets. Spherical C.sub.60 adds a new dimension to the chemistry of aromatic compounds.
C.sub.60 has been shown to be one of the most chemically versatile molecules known. Chemists have produced fullerene derivatives by adding carbons to the C.sub.60 sphere while maintaining (in some instances) the aromatic electron structure. C.sub.60 has been reported to readily accept free radicals and may be useful in polymerization processes. Other potential applications include commercial basics such as catalysis as well as superconductivity and ferromagnetism. C.sub.60 easily accepts electrons. Solid fullerene can also be doped with an alkali metal such as potassium, and these compounds are called fullerides. Although pure C.sub.60 is an insulator, some fullerides are semiconductors or superconductors; for example, K.sub.3 C.sub.60 is a superconductor.
C.sub.60, a naturally hollow molecule, has been proposed as a host for nuclides. In addition, unmodified fullerenes are insoluble in water, suggesting that they may react very little with biological tissue. In the presence of light and oxygen, the C.sub.60 molecule can pass its excitation energy onto nearby oxygen molecules, creating a short-lived, reactive form of oxygen called singlet oxygen.